How can I test for two field values in an SSRS Textbox's Expression property?

I successfully use this Expression in a Textbox:

=IIF((Fields!Week.Value="WK1"),Fields!Price.Value,"")

I need to be more granular, though - not only am I looking for a Week value of "WK1" for this particular column before assigning it the value of the Price field, but also based on the value of the Unit field.

...but when I try previewing the results after either of those, I get, "The report preview failed because the report could not be built. Read the errors, warnings, and messages in the Error List window for specific build failures."